1. Field of the Invention
This invention relates to a hoist having overload safety means for preventing lifting, lowering and dragging under overload condition and capable of securely lowering a load irrespective of its weight.
2. Description of the Prior Art
With hoists such as electric chain blocks whose load sheave is driven through reduction gears by electric motors, it has been proposed to provide on a driven shaft of a reduction gear mechanism a brake assembly consisting of a brake support member, a brake retainer member, a brake ratchet wheel and friction plates between the members and further provide an overload safety device including frictional connections interposed between the brake retainer member and a driven gear fitted thereon to prevent lifting of a load by slippage in the overload safety device when the load sheave is subjected to an overload.
With this such as electric hoist, however, the friction transmission force of the overload safety device often becomes less than that of the brake assembly owing to its overtightening, so that once lifted the load cannot be lowered due to slippage in the overload safety device.
In order to solve this problem, it has been proposed to interpose a one-way clutch between the driven gear and the brake retainer member to render inoperative the overload safety device when lowering the load, as disclosed for example in Japanese Patent Application Publication No. 42,937/78. In this case, however, as the overload cannot be prevented in lowering the load, it is indispensably required to provide a limit switch for detecting a lowermost position of the load in view of the safety in operation which would possibly make the hoist complicated.
Moreover, another type of hoist having an overload safety device has been disclosed in U.S. Pat. No. 3,770,086 which overload safety device comprises a driving member threadedly engaged with a driving shaft and formed with a number of bottomed radial recesses opening at the outer periphery of the driving member, a member arranged about the driving member, fixed to a hand wheel and provided with a number of cuts each having a gentle slope surface in one direction in opposition to the above bottomed recess, and rollers and springs accommodated in the bottomed recesses so as to urge the rollers toward the cuts of the member by the springs to provide a difference in torque for winding-up and winding-off a load. With such a hoist, however, the number of rollers received in the number (more than ten) of recesses and urged toward the cuts by the springs make complicated the construction of the hoist and render difficult its assembling and adjustment of the spring urging force so as not to be suitable for mass-production.